This invention relates to a loop network system comprising a plurality of stations connected by a loop-shaped transmission path so as to carry out unidirectional communication through the transmission path.
In order to improve efficiency in business, attempts have been directed to development of a local area network communication system which is capable of compatibly carrying out a wide variety of communication, such as voice, data, and video communication. More particularly, the network is capable of selectively carrying out a communication of a wide variety. A loop network system is typical of the local area network communication system.
A conventional loop network system of the type described is proposed by Takashi Matsuda et al in Proceeding of National Telecommunications Conference (1981) of IEEE, pp. G1.2.1-G1.2.5, titled "General purpose local network using optical loop highway" and comprises a center terminal unit and a plurality of remote terminal units connected through a loop of an optical fiber to the center terminal unit. Communication is carried out between the units in a time division fashion through a single time slot assigned in each frame for the communication. The conventional system is suitable for synchronous communication, such as voice communication, because each time slot is fixedly kept until completion of the communication. However, the system is not convenient for video communication for which a plurality of time slots are necessary in each frame.
Another loop network system is described by Werner Bux et al in Proceeding of National Telecommunications Conference (1981), pp. A2.2.1-A2.2.6 and comprises a control station and the other stations (to be called node stations) connected by a loop-shaped transmission path of an optical fiber to the control station. Each node station accommodates at least one terminal equipment. With this system, communication is carried out between the stations by the use of a token for controlling access to the transmission path. More specifically, each station is qualified for communication when the token is seized or kept by the station. Each station continues communication during seizure of the token and sends the token downstream through the transmission path. It is therefore possible with this system to transmit a signal block or packet having various lengths and to increase an amount of information to be transmitted between the stations.
Werner et al also describe a method of making such a token carry a degree of priority given to information to be transmitted. This method enables synchronous communication of a voice or audio signal or the like by assigning a higher degree of priority to the synchronous communication as compared with asynchronous communication of, for example, a data signal. In this method, such a voice signal can be transmitted at every predetermined period to the transmission path in preference to the data signal when each node station seizes or acquires a reservation right which is specified by the higher degree of priority to guarantee preferential transmission of a predetermined amount of information.
With this system, each node station should communicate with the control station so as to seize the reservation just prior to transmission of information. Therefore, the control station must have various kinds of functions to monitor traffic on the transmission path, to detect whether or not a reservation right is to be assigned to each node station, and to communicate with each node station. This means that the functions of the system concentrate on the control station and that the control station is intricate in structure. When faults occur in the control station, the whole of the system is adversely affected by the faults.
In addition, each station should not transfer or release the token downstream until a whole signal block which is sent from each station to the transmission path is returned back to the station in question through the transmission path. Otherwise, a downstream one of the stations cannot detect completion of information in the station under consideration even when the token is received at the downstream station. As a result, the efficiency of transmission is objectionably reduced in a network of a high speed transmission.